1. Field of the Invention
This invention concerns improvements relating to the monitoring or determination of the permeability of sheet materials, particularly but not exclusively webs of paper used in the tobacco industry.
2. Brief Description of the Prior Art
In United Kingdom Patent Specification No. 1,239,408, it has been proposed that, in the manufacturing of cigarettes, the permeability of a web of cigarette paper should be monitored continuously for the purpose of identifying any portions of the web which are unsatisfactory. A device disclosed in that specification for monitoring paper web comprises a suction chamber disposed in contact with one side of the web and a second chamber disposed in contact with the other side thereof. A bleed hole communicates with the second chamber, whereby ambient air may flow into the second chamber when a partial vacuum is established in the suction chamber. Pressure indicating means is operative to measure the pressure in the second chamber, the pressure therein varying in accordance with variations in the permeability of the web which is conveyed between the two chambers. The use of the partial vacuum is disadvantageous in that paper dust and other debris are likely to be sucked into and cause malfunctioning of the device. The monitoring device of the aforesaid Specification also has the disadvantage that both chambers have to be maintained in contact with respective sides of the web in order to produce necessary hermetic seals therewith.
To our knowledge, there is no device currently available which adequately meets the practical requirements for continuous on-line monitoring of paper webs of the tobacco industry, such webs including, for example, webs of tipping, filter-plug wrapping and cigarette wrapping material.